As one of an access network that connects homes or companies to a higher-order network, there has been used a PON (passive optical network) system for connecting a master station device (hereinafter, OLT (optical line terminal)) and many slave station devices (hereinafter, ONUs (optical network units)) in a one-to-many relationship by an optical fiber and a splitter. In such a PON system with a one-to-many relationship, when upstream data communication from each of the ONUs to an OLT is performed, each ONU transmits a bandwidth request signal indicating a bandwidth request amount for its own device to the OLT, the OLT allocates a bandwidth to each of the ONUs based on the bandwidth request amount of each of the ONUs, and transmits a transmission permission signal indicating a transmission start time and a transmission time to each of the ONUs. Thereafter, each of the ONUs receives the transmission permission signal transmitted from the OLT for its own device, and performs a bandwidth allocating process for transmitting upstream data according to the content of the transmission permission signal.
As such a PON system, there has been known a system mode having a data transmission rate of 1 Gbit/s respectively for upstream communication and downstream communication, in which bi-directional communication by wavelength multiplexing using different wavelengths for upstream communication and downstream communication is performed, and when a plurality of ONUs transmit upstream data, the data is transmitted according to a time-division multiplexing system in which an upstream bandwidth is divided among the plurality of ONUs (see, for example, Nonpatent Literature 1).
Meanwhile, a PON system that realizes a high data transmission rate has been developed with performance improvements of optical devices. The PON system has a configuration capable of performing transmission at a high data transmission rate in the same PON system, while maintaining upward compatibility with an existing PON system having a low data transmission rate. As a developed model of such a PON system, there has been known another PON system in which an OLT has data transmission rates of 1 Gbit/s and 10 Gbit/s in both upstream and downstream directions, and as an ONU, a device having a data transmission rate of 1 Gbit/s in both upstream and downstream directions, a device having a data transmission rate of 10 Gbit/s in both upstream and downstream directions, and a device having a data transmission rate of 1 Gbit/s in the upstream direction and 10 Gbit/s for the downstream direction are present in a mixed manner (see, for example, Nonpatent Literature 2). In such a PON system in which a plurality of ONUs having different data transmission rates are connected, data transmission is performed according to a wavelength multiplexing system in a downstream direction by dividing a wavelength for each of data transmission rates, and in an upstream direction, data transmission is performed according to a time-division multiplexing system in which one wavelength is used and the bandwidth is shared by a plurality of ONUs. Therefore, upstream bandwidth allocation needs to be performed also in a communication system including ONUs having a plurality of data transmission rates.
Further, as a method of allocating an upstream bandwidth to ONUs by an OLT, there has been known a technique in which each ONU informs the OLT of two values of large and small as a bandwidth request amount, the OLT allocates a small bandwidth request amount to the ONU in a low delay class in a short cycle, and allocates a large bandwidth request amount to the ONU in a normal class in a long cycle, thereby reducing a delay of upstream data from the ONUs and improving the bandwidth use efficiency (see, for example, Nonpatent Literature 3).    Nonpatent Literature 1: “IEEE Standards 802.3ahTM-2004”, 7 Sep. 2004, IEEE (Institute of Electrical and Electronics Engineers) Computer Society    Nonpatent Literature 2: Glen Kramer et al., “10GEPON-1GEPON Coexistence”, Jan. 15, 2007, IEEE802.3av 10GEPON Task Force, Presentation Materials    Nonpatent Literature 3: Osamu Yoshihara et al., “Dynamic Bandwidth Allocation Algorithm for GE-PON”, Technical Report of IEICE, NS2002-17, the Electronics, Information and Communication Engineers